Metal sheet material having an enamel coating on one surface is used extensively in the formation of beverage containers. Soft drink beverages are capable of reacting with an exposed metal surface, such as an aluminum surface. Thus, the surface of the container which is contacted by the beverage must include a protective coating to prevent an undesired reaction between the beverage and the container surface.
A can end for a beverage container is customarily scored to define a line of weakness in the can end. During usage of the can, the line of weakness in the can end may be ruptured by a rupturing tool which applies a localized high force to the can end at the line of weakness. The rupturing tool may take the form of an opening tab which is physically secured to the can end. A conventional means of securing an opening tab to a can end involves the formation of an upstanding hollow rivet from the thin metal of the can end, which rivet may then be positioned within an aperture in the opening tab. The hollow rivet may then be compressed to form an enlargment at the outer end of the rivet to secure the opening tab to the can end by enlargement of the rivet.
With an opening tab, thus, secured to the can end, the movement of the opening tab relative to the can end may apply a rupturing force to the can end along the line of weakness therein. As the can end is, thereby, ruptured, an opening may be provided in the can end. The nature of the opening and its form relative to the can end in many instances is determined by the nature of the product within the container. If, for example, the product is a liquid beverage, the opening formed in the can end may provide an opening for pouring the beverage from the container. Conversely, if the product within the container is a solid, the opening formed in the can end may encompass almost the entire can end. Such a can end is commonly termed a "full-panel pull-out," and may be used, for example, with a wide variety of products, such as potato chips, party dips, and the like.
With the emphasis now placed on ecology, can ends with removable tear strips have received a great deal of criticism. In fact, in some states, beverage containers having removable tear strips have been outlawed because of fears that users of the containers would create objectionable litter by leaving removable tear strips in public places, such as on beaches, etc., where they could be a possible source of injury.
Because of the emphasis on ecology, can ends are now being constructed which have tear strips that are not removable from the can end. In use of such ends, the tear strip, after being broken away from the can end along the line of weakness, still remains connected to the can end through a connecting strip. A free end of the tear strip may then be forced through the opening in the can end that is formed by severing the line of weakness such that the tear strip then extends into the interior of the can where it is out of the way and does not interfere with the use of the can.
In any of the foregoing can end constructions, the interior surface of the can and the can end may have a protective coating thereon if the contents of the can are reactive with an exposed metal surface of the can. As stated, this is particularly necessary in cans which contain soft drink beverages. However, the scoring of a metal sheet material having a protective coating during the formation of a can end may present considerable problems.
During scoring, the protective coating on the interior or non-public surface of the can end may be broken. When this occurs, the contents of the container may then be able to react with the exposed metal surface caused by the break in the protective coating. This may cause spoilage of the can contents which may cause considerable loss to the manufacturer in recalling the cans with spoiled product from the inventories of the merchants in the distribution chain for the product. Such an occurrence could be disastrous to the manufacturer. Thus, it is imperative that the protective coating on the non-public side of the can end remain intact during the formation of the can end.
To insure that the protective coating on the interior surface of the can end is intact after scoring the can end to create a line of weakness therein, it has been necessary, in some instances, to resurface the non-public side of the can end with a new protective coating after scoring the can end. This may be an expensive operation since individual can ends may be more difficult to process during coating than a sheet of metal as used in forming the can ends. Thus, the formation of a protective coating on the non-public side of the can ends after scoring of the ends is not a satisfactory solution to the problem of insuring the presence of a complete protective coating on the non-public surfaces of the can ends. However, at the same time, the potential economic impact on a manufacturer of having to recall cans because of spoilage caused by reaction of the product with exposed metal at a break in the protective coating poses a risk which is not acceptable.
In view of the above difficulties, it would be desirable if a procedure could be devised for forming a line of weakness in a metal sheet with a protective coating thereon without causing a break in the protective coating at the line of weakness. This would permit the formation of can ends in a more direct and inexpensive manner without having to resurface the can ends after completing the metal-forming operations thereon. Also, such a procedure would substantially eliminate the risk of causing a break in the protective coating on the interior surface of the can ends during formation of a line of weakness in the can ends.